A vacuum circuit breaker includes a stationary contact and a moving contact that is placed opposite the stationary contact. The stationary contact and the moving contact are placed in a vacuum chamber in which vacuum is maintained around the contacts. With the contacts of the vacuum circuit breaker closed to cause current to flow in a main circuit conductor, when the contacts of the vacuum circuit breaker are opened to interrupt current flowing in the main circuit, if the vacuum degree in the vacuum chamber is high, high arc-suppression capability of vacuum causes current to be interrupted. However, if the vacuum degree in the vacuum chamber is reduced due to factors such as a crack in the vacuum chamber, discharge of gas molecule adsorbed to a metal or insulator, as well as transmission of atmospheric gas, opening the contacts may cause breakdown to prevent the interruption of current, which may damage the equipment in the worst case. Thus, in order to know what is going on in the vacuum circuit breaker without damaging the breaker and its peripheral devices, a vacuum degree deterioration monitoring device for determining whether or not the vacuum degree in the vacuum chamber has deteriorated has been under development.
For example, in existing literatures, a method is disclosed in which the normality of the vacuum degree is determined by detecting, using an antenna mounted in the breaker, electromagnetic wave due to discharge caused by vacuum degree deterioration resulting in withstand voltage reduction (PTL 1). Furthermore, in one discharge detection method, a metal electrode placed in a bushing is used as an antenna to allow a signal having a wide frequency range to be detected (PTL 2). Furthermore, a method is described in which a discharge location is detected using the time difference of a signal received by two or more antennas mounted in the breaker (PTL 3).
Furthermore, a method is described in which a partial discharge occurring area is located by placing discharge detectors at two spots in a gas isolation switching device and comparing the outputs of the detectors (PTL 4).